U.S. patent application number 15/432290 was filed with the patent office on 2017-08-17 for wavelength tunable laser module and method of controlling wavelength thereof.
This patent application is currently assigned to FURUKAWA ELECTRIC CO., LTD.. The applicant listed for this patent is FURUKAWA ELECTRIC CO., LTD.. Invention is credited to Kengo MURANUSHI, Masayoshi NISHITA, Atsushi YAMAMOTO.
Application Number | 20170237499 15/432290 |
Document ID | / |
Family ID | 59561882 |
Filed Date | 2017-08-17 |
United States Patent
Application |
20170237499 |
Kind Code |
A1 |
NISHITA; Masayoshi ; et
al. |
August 17, 2017 |
WAVELENGTH TUNABLE LASER MODULE AND METHOD OF CONTROLLING
WAVELENGTH THEREOF
Abstract
A method of controlling a wavelength of a wavelength tunable
laser module includes: referring to data of measured frequencies
and wavelength filter control values at two or more points for each
basic frequency channel, the data being stored in a memory of a
controller; selecting the basic frequency channel closest to a
frequency of laser light that a laser light source is instructed to
emit; calculating a first wavelength filter control value for
providing the instructed frequency of laser light from the data of
the measured frequencies allocated to the basic frequency channel
closest to the instructed frequency and the wavelength filter
control values; and controlling the transmission characteristic of
a wavelength filter using the first wavelength filter control
value.
Inventors: |
NISHITA; Masayoshi; (Tokyo,
JP) ; MURANUSHI; Kengo; (Tokyo, JP) ;
YAMAMOTO; Atsushi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FURUKAWA ELECTRIC CO., LTD. |
Tokyo |
|
JP |
|
|
Assignee: |
FURUKAWA ELECTRIC CO., LTD.
Tokyo
JP
|
Family ID: |
59561882 |
Appl. No.: |
15/432290 |
Filed: |
February 14, 2017 |
Current U.S.
Class: |
398/90 |
Current CPC
Class: |
H04J 14/0227 20130101;
H04J 14/0224 20130101; H04J 14/0226 20130101; H04B 10/503 20130101;
H04B 10/572 20130101; H04B 10/506 20130101 |
International
Class: |
H04B 10/50 20060101
H04B010/50 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 15, 2016 |
JP |
2016-025479 |
Claims
1. A method of controlling a wavelength of a wavelength tunable
laser module that includes a laser light source that emits laser
light, a wavelength filter having a periodic transmission
characteristic with respect to a wavelength of light, and a
controller that controls a wavelength of laser light emitted from
the laser light source on the basis of power of the laser light
transmitted by the wavelength filter and controls the transmission
characteristic of the wavelength filter, the method comprising:
referring to data of measured frequencies and wavelength filter
control values at two or more points for each basic frequency
channel, the data being stored in a memory of the controller;
selecting the basic frequency channel closest to a frequency of
laser light that the laser light source is instructed to emit;
calculating a first wavelength filter control value for providing
the instructed frequency of laser light from the data of the
measured frequencies allocated to the basic frequency channel
closest to the instructed frequency and the wavelength filter
control values; and controlling the transmission characteristic of
the wavelength filter using the first wavelength filter, control
value.
2. The method of controlling a wavelength of a wavelength tunable
laser module according to claim 1, wherein, at the calculating, the
first wavelength filter control value for providing the instructed
frequency is calculated by a linear approximation or a second or
higher order polynomial approximation from the data of the measured
frequencies interposing the instructed frequency therebetween and
the wavelength filter control values.
3. The method of controlling a wavelength of a wavelength tunable
laser module according to claim 1, wherein, at the calculating, the
first wavelength filter control value for providing the instructed
frequency is calculated by a linear approximation or a second or
higher order polynomial approximation from the data of the
measurement frequencies that do not interpose the instructed
frequency therebetween and the wavelength filter control
values.
4. The method of controlling a wavelength of a wavelength tunable
laser module according to claim 1, wherein the data of the measured
frequencies and the wavelength filter control values that are
stored in the memory of the controller are preliminarily recorded
in the memory as a result of additional measurement, the additional
measurement being performed by dividing frequency spacing between
adjacent basic frequency channels into n divisions and performing
measurement on a frequency of at least one of (n-1) divided points
of the n divisions.
5. The method of controlling a wavelength of a wavelength tunable
laser module according to claim 1, further comprising: selecting
the basic frequency channel secondly closest to the frequency of
laser light that the laser light source is instructed to emit;
calculating a second wavelength filter control value for providing
the instructed frequency of laser light from the data of the
measurement frequencies allocated to the basic frequency channel
secondly closest to the instructed frequency and the wavelength
filter control values; and controlling the transmission
characteristic of the wavelength filter using one closer to a
center value of the wavelength filter control value stored in the
memory out of the first wavelength control value and the second
wavelength control value instead of the first wavelength control
value.
6. A wavelength tunable laser module comprising: a laser light
source that emits laser light; a wavelength filter that has a
periodic transmission characteristic with respect to a wavelength
of light; a controller that controls a wavelength of laser light
emitted from the laser light source on the basis of power of the
laser light transmitted by the wavelength filter and controls the
transmission characteristic of the wavelength filter, wherein the
controller includes a computing unit that is programmed so as to
execute: referring to data of measured frequencies and wavelength
filter control values at two or more points for each basic
frequency channel, the data being stored in a memory of the
controller; selecting the basic frequency channel closest to a
frequency of laser light that the laser light source is instructed
to emit; calculating a first wavelength filter control value for
providing the instructed frequency of laser light from data of the
measurement frequencies allocated to the basic frequency channel
closest to the instructed frequency and the wavelength filter
control values; and controlling the transmission characteristic of
the wavelength filter using the first wavelength filter control
value.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to and incorporates
by reference the entire contents of Japanese Patent Application No.
2016-025479 filed in Japan on Feb. 15, 2016.
BACKGROUND
[0002] The present disclosure relates to a wavelength tunable laser
module and a method of controlling a wavelength of the wavelength
tunable laser module.
[0003] With an increase in amount of information communication,
optical signals are demanded to be multiplexed with narrower
wavelength spacing in a wavelength division multiplexing (WDM)
communication field in which a plurality of optical signals having
different: wavelengths are multiplexed and transmitted
simultaneously in a single optical fiber. For multiplexing the
optical signals with narrower wavelength spacing, it is necessary
to highly accurately control a wavelength of laser light emitted
from a laser element as a signal.
[0004] In recent years, what is called a flexible grid method is
introduced into the frequency spacing of laser light to efficiently
use an optical transmission wavelength bandwidth instead of a
common fixed grid method in which the frequency spacing is fixed to
25 GHz or 50 GHz. The flexible gird method allows wavelengths to be
arranged randomly with different frequency spacing.
[0005] It has been examined to utilize a semiconductor laser module
using an etalon filter for communication employing the flexible
grid method. The etalon filter has a wavelength transmission
characteristic changeable by controlling a temperature thereof. The
semiconductor laser module using the etalon filter splits a part of
laser light emitted from a semiconductor laser element to the
etalon filter and controls a temperature of the semiconductor laser
element on the basis of power of the split light transmitted by the
etalon filter, thereby controlling the wavelength of laser light
emitted from the semiconductor laser element. The etalon filter has
a periodic wavelength transmission characteristic. The periodic
wavelength transmission characteristic is shifted in a wavelength
direction depending on the temperature of the etalon filter. As a
result, the semiconductor laser module using the etalon filter may
control laser light emitted therefrom such that laser light has
desirable wavelength and power. For example, refer to Japanese
Patent Application Laid-open No. 2012-33895.
[0006] When the flexible grid method is achieved on the basis of
the extension of the related technique, the flexible grid method
has a limit. For example, hen the whole of C-band is covered by
channels based on a common frequency grid with 50 GHz spacing, the
number of channels is approximately 100. When the related technique
is simply extended to a frequency grid with spacing of 0.1 GHz,
which is an example of the grid spacing of the wavelength tunable
laser module for providing the flexible grid method, the number of
channels is approximately 50,000, which is 500 times of that of the
50 GHz grid. It is, however, not practical, from mass production
and cost viewpoints, that calibration is performed on all of the
approximately 50,000 channels and laser driving conditions and
control target values as the results of the calibration are stored
in a memory.
SUMMARY
[0007] It is an object of the present disclosure to at least
partially solve the problems in the related technology.
[0008] According to one aspect of the present disclosure, there is
provided a method of controlling a wavelength of a wavelength
tunable laser module that includes a laser light source that emits
laser light, a wavelength filter having a periodic transmission
characteristic with respect to a wavelength of light, and a
controller that controls a wavelength of laser light emitted from
the laser light source on the basis of power of the laser light
transmitted by the wavelength filter and controls the transmission
characteristic of the wavelength filter, the method including:
referring to data of measured frequencies and wavelength filter
control values at two or more points for each basic frequency
channel, the data being stored in a memory of the controller;
selecting the basic frequency channel closest to a frequency of
laser light that the laser light source is instructed to emit;
calculating a first wavelength filter control value for providing
the instructed frequency of laser light from the data of the
measured frequencies allocated to the basic frequency channel
closest to the instructed frequency and the wavelength filter
control values; and controlling the transmission characteristic of
the wavelength filter using the first wavelength filter control
value.
[0009] According to another aspect of the present disclosure, there
is provided a wavelength tunable laser module including: a laser
light source that emits laser light; a wavelength filter that has a
periodic transmission characteristic with respect to a wavelength
of light; a controller that controls a wavelength of laser light
emitted from the laser light source on the basis of power of the
laser light transmitted by the wavelength filter and controls the
transmission characteristic of the wavelength filter, wherein the
controller includes a computing unit that is programmed so as to
execute: referring to data of measured frequencies and wavelength
filter control values at two or more points for each basic
frequency channel, the data being stored in a memory of the
controller; selecting the basic frequency channel closest to a
frequency of laser light that the laser light source is instructed
to emit; calculating a first wavelength filter control value for
providing the instructed frequency of laser light from data of the
measurement frequencies allocated to the basic frequency channel
closest to the instructed frequency and the wavelength filter
control values; and controlling the transmission characteristic of
the wavelength filter using the first wavelength filter control
value.
[0010] The above and other objects, features, advantages and
technical and industrial significance of this disclosure will be
better understood by reading the following detailed description of
presently preferred embodiments of the disclosure, when considered
in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a schematic diagram illustrating an exemplary
structure of a wavelength tunable laser module;
[0012] FIG. 2 is a graph illustrating an exemplary periodic
transmission characteristic of a wavelength filter;
[0013] FIG. 3 is a graph illustrating exemplary PD current ratios
at two wavelength filter temperatures;
[0014] FIG. 4A is a schematic diagram illustrating a relation
between basic frequency channels and additional measurement
frequencies;
[0015] FIG. 4B is a schematic diagram illustrating a relation
between basic frequency channels and additional measurement
frequencies;
[0016] FIG. 5 is a schematic diagram explaining an effect of
calculation method 2;
[0017] FIG. 6 is a schematic diagram illustrating an example of the
whole flow of a control method of the wavelength tunable laser
module;
[0018] FIG. 7 is a schematic diagram illustrating an example of
shifting a discrimination curve in a wavelength direction;
[0019] FIG. 8 is a graph illustrating a relation between a current
supplied to a semiconductor optical amplifier (SOA) and a current
output by a power monitor;
[0020] FIG. 9 is a schematic diagram illustrating an example of the
discrimination curve shifted so as to correspond to an instructed
frequency;
[0021] FIG. 10 is a graph illustrating an exemplary measurement
result of wavelength accuracies in the wavelength control method of
the wavelength tunable laser module according to the embodiment;
and
[0022] FIG. 11 is a graph illustrating an exemplary measurement
result of wavelength accuracies in the wavelength control method of
the wavelength tunable laser module according to a comparative
example.
DETAILED DESCRIPTION
[0023] The following describes a method of controlling a wavelength
of a wavelength tunable laser module according to an embodiment of
the disclosure in detail with reference to the accompanying
drawings. The flowing embodiment does not limit the disclosure. In
the drawings, the same or corresponding components are labeled with
the same reference numerals. The drawings are schematic and thus it
is noted that thicknesses of respective layers and ratios between
the thicknesses of the respective layers differ from those of the
actual components. Furthermore, the relation and ratios between
dimensions of the respective components may differ from one another
among the drawings.
Embodiment
[0024] FIG. 1 is a schematic diagram illustrating an exemplary
structure of a wavelength tunable laser module. The wavelength
tunable laser module illustrated in FIG. typical example of an
apparatus used for performing the method of controlling the
wavelength of the wavelength tunable laser module according to the
embodiment.
[0025] As illustrated in FIG. 1, this wavelength tunable laser
module 100 includes a wavelength tunable light source unit 200 and
a controller 300 as major components. The wavelength tunable light
source unit 200 outputs laser light having desired wavelength and
power in accordance with control from the controller 300, and
supplies the laser light to an apparatus in a following stage. The
controller 300 is connected to a higher-level controller provided
with a user interface, for example. The controller 300 controls the
wavelength tunable light source unit 200 in accordance with the
user's instruction via the higher-level controller. The wavelength
tunable laser module 100 may include the wavelength tunable light
source unit 200 and the controller 300 individually or by
assembling them on a circuit substrate together.
[0026] As illustrated in FIG. 1, the wavelength tunable light
source unit 200 includes a laser light source 210, a wavelength
detector 220, an optical demultiplexer 230, a power monitor 240,
and an optical fiber 260.
[0027] The laser light source 210 includes distributed feedback
laser diodes (DFB-LDs) 211, an optical waveguide 212, an optical
multiplexer 213, a semiconductor optical amplifier (SOA) 214, a
Peltier element 215, and a laser temperature monitoring element
216. The DFB-LDs 211, the optical waveguide 212, the optical
multiplexer 213, and the SOA 214 may be formed on a single
semiconductor chip.
[0028] The laser light source 210 includes a plurality of DFB-LDs
211 that are arranged in a stripe shape and emit laser light having
wavelengths different from one another from respective front facets
thereof. An emission wavelength of each DFB-LD 211 may be
controlled by adjusting the temperature of the DFB-LD 211. The
respective DFB-LDs 211 are placed on the Peltier element 215. The
temperature of the DFB-LD 211 may be changed by the Peltier element
215. The laser temperature monitoring element 216 is provided on
the Peltier element 215, thereby making it possible to monitor the
temperature of the DFB-LL) 211.
[0029] The emission wavelength of the DFB-LD is changeable in an
approximate range from 3 nm to 4 nm. The emission wavelengths of
the respective DFB-LDs 211 are designed such that the emission
wavelengths of the DFB-LDs 211 have spacing of an approximate range
from 3 nm to 4 nm therebetween. The laser light source 210 selects
and drives one suitable for obtaining a desired wavelength of laser
light out of the multiple DFB-LDs 211, and controls the temperature
of the selected DFB-LD 211, thereby making it possible to emit
laser light over a continuous wavelength bandwidth wider than that
of a single DFB-LD.
[0030] In order to cover the whole wavelength bandwidth for WDM
communication (e.g., C-band (1.53 .mu.m to 1.56 .mu.m) or L-band
(1.57 .mu.m to 1.61 .mu.m)), 12 DFB-LDs 211 are integrated each of
which is capable of changing its emission wavelength in a range
from 3 nm to 4 nm, for example. As a result, the wavelength of
laser light is changeable over a wavelength bandwidth equal to or
larger than 30 nm.
[0031] Laser light emitted from any of the DFB-LDs 211, travels the
optical waveguide 212 and the optical multiplexer 213, thereafter,
is guided to one optical path, amplified by the SOA 214, and
emitted from the laser light source 210.
[0032] The optical demultiplexer 230 is optically coupled to the
optical fiber 260 so as to output, from the wavelength tunable
light source unit 200, most of laser light output from the laser
light source 210. Simultaneously, the optical demultiplexer 230
splits a part of laser light output from the laser light source 210
into the power monitor 240 and the wavelength detector 220 to
supply the split light to the power monitor 240 and the wavelength
detector 220. The optical fiber 260 outputs the laser light
received from the optical demultiplexer 230 to supply the received
light to the apparatus in the following stage (not
illustrated).
[0033] The power monitor 240 is a measuring instrument using a
photo diode that outputs an electrical signal according to
intensity (power) of received light. The electrical signal output
from the power monitor 240, thus, may be converted into the power
of laser light output from the optical fiber 260.
[0034] The wavelength detector 220 includes a wavelength filter
221, a Peltier element 222, a filter temperature monitoring element
223, and a wavelength monitor 250. The wavelength filter 221 has a
periodic transmission characteristic with respect to the wavelength
of light. An etalon filter is used for the wavelength filter 221,
for example. FIG. 2 is a graph illustrating an exemplary periodic
transmission characteristic of the wavelength filter.
[0035] The periodic wavelength transmission characteristic of the
wavelength filter 221 is shifted in a wavelength direction
depending on the temperature of the wavelength filter 221. A
temperature coefficient of the shifting differs from material to
material forming the wavelength filter 221. The temperature
characteristic of the etalon filter formed by quartz (SiO.sub.2) is
approximately 1.25 GHz/.degree. C. The temperature characteristic
of the etalon filter formed by crystal is approximately 1.9
GHz/.degree. C. The temperature characteristic of the etalon filter
formed by bismuth germanium oxide (Bi.sub.12GeO.sub.20:BGO) is
approximately 2.5 GHz/.degree. C.
[0036] When the temperature characteristic of the etalon filter is
too large, the transmission characteristic of the etalon filter
becomes sensitive to a change in temperature. As a result, the
transmission characteristic of the etalon filter easily varies. In
contrast, when the temperature characteristic of the etalon filter
is too small, the temperature of the etalon filter needs to be
largely shifted in controlling output of a use frequency other than
a basic frequency channel. As a result, power consumption is
increased.
[0037] The wavelength filter 221 is placed on the Peltier element
222. The temperature of the wavelength filter 221 may be changed by
the Peltier element 222. The filter temperature monitoring element
223 is provided on the Peltier element 222, thereby making it
possible to monitor the temperature of the wavelength filter
221.
[0038] Laser light split by the optical demultiplexer 230 is
transmitted by the wavelength filter 221 of the wavelength detector
220 and incident on the wavelength monitor 250. The wavelength
monitor 250 is a measuring instrument using a photo diode that
outputs an electrical signal according to intensity (power) of
received light in the same manner as the power monitor 240. The
electrical signal output from the wavelength monitor 250 is a
result of multiplying the power of laser light that is output from
the laser light source 210, split by the optical demultiplexer 230,
and incident on the wavelength filter 221 by the transmission
characteristic illustrated in FIG. 2.
[0039] As described above, the wavelength filter 221 has a periodic
transmission characteristic with respect to the wavelength of
light. Let a ratio of the electrical signal (PD2) output from the
wavelength monitor 250 to the electrical signal (PD1) output from
the power monitor 240 be a PD current ratio. The PD current ratio
(PD2/PD1) also has a periodic value with respect to the wavelength
of light. The periodic wavelength transmission characteristic of
the wavelength filter 221 is shifted in the wavelength direction
depending on the temperature of the wavelength filter 221. FIG. 3
is a graph illustrating exemplary PD current ratios at two
temperatures of the wavelength filter 221.
[0040] The curves in the graph illustrated in FIG. 3, which are
called discrimination curves, each illustrate a relation between
the measured PD current ratio and the wavelength of output laser
light. When the PD current ratio is monitored using such a
discrimination curve illustrated in FIG. 3, an error in the
wavelength of laser light output from the laser light source 210
may be detected. The discrimination curve of the wavelength filter
221 may be shifted in the wavelength direction by controlling the
temperature of the wavelength filter 221. The discrimination curve,
thus, may be obtained that corresponds to a desired wavelength of
laser light to be output from the laser light source 210.
[0041] The following describes a structure of the controller 300.
The structure of the controller 300 illustrated in FIG. 1
illustrates functions as blocks. The blocks illustrated in FIG. 1
are, however, not physically separated from one another.
[0042] As illustrated in FIG. 1, the controller 300 includes a
DFB-LD selection circuit 311, a DFB-LD current control circuit 312,
a laser temperature monitoring circuit 321, a laser temperature
control circuit 322, an SOA control circuit 330, a PD1 current
monitoring circuit 341, a PD2 current monitoring circuit 342, an
etalon temperature monitoring circuit 351, an etalon temperature
control circuit 352, a digital computing unit 360, and a memory
370.
[0043] The DFB-LD selection circuit 311 selects one of the DFB-LDs
211 in accordance with an instruction from the digital computing
unit 360. Specifically, the selection may be achieved by switching
switches of circuits each supplying a current to the corresponding
DFB-LD 211. The DFB-LD current control circuit 312 controls the
current supplied to the DFB LD 211 in accordance with an
instruction from the digital computing unit 360.
[0044] The laser temperature monitoring circuit 321 detects the
temperature of the laser temperature monitoring element 216 to
identify the temperature of the DFB-LD 211, and transmits the
temperature of the DFB-LD 211 to the digital computing unit 360 as
a digital signal. The laser temperature control circuit 322
controls a current supplied to the Peltier element 215 such that
the temperature of the DFB-LD 211 becomes the temperature
instructed from the digital computing unit 360.
[0045] The SOA control circuit 330 controls a current supplied to
the SOA 214 to adjust a gain of the SOA 214 in accordance with an
instruction from the digital computing unit 360.
[0046] The PD1 current monitoring circuit 341 converts the current
output from the power monitor 240 into a digital signal and
transmits the digital signal to the digital computing unit 360. The
PD2 current monitoring circuit 342 converts the current output from
the wavelength monitor 250 into a digital signal and transmits the
digital signal to the digital computing unit 360.
[0047] The etalon temperature monitoring circuit 351 detects the
temperature of the filter temperature monitoring element 223 to
identify the temperature of the wavelength filter 221, and
transmits the temperature of the wavelength filter 221 to the
digital computing unit 360 as a digital signal. The etalon
temperature control circuit 352 controls a current supplied to the
Peltier element 222 such that the temperature of the wavelength
filter 221 becomes the temperature instructed from the digital
computing unit 360.
[0048] The memory 370 stores therein various types of data such as
laser driving conditions and target values of the wavelength
detector necessary for the digital computing unit 360 to calculate
control parameters. Particularly, the memory 370 stores therein a
set of the frequency of the basic frequency channel and the
temperature of the wavelength filter 221 for each basic frequency
channel, and a set of a measured frequency at an additional
measurement frequency and the temperature of the wavelength filter
221.
[0049] The basic frequency channel is a parameter that may be set
in accordance with a design concept. The basic frequency channel
may use the frequency determined by a standard, for example. The
spacing between the basic frequency channels may use frequency
spacing such as a half period or a quarter period of the
transmission characteristic of the wavelength filter 221. The
spacing between the basic frequency channels is set to be larger
than minimum spacing between frequencies of laser light that the
wavelength tunable light source unit 200 may output by automatic
frequency control (AFC) performed by the controller 300. When the
spacing between the basic frequency channels is smaller than the
minimum spacing between frequencies of laser light that the
wavelength tunable light source unit 200 may output by the AFC
performed by the controller 300, parameters for all of the
frequencies capable of being output need to be stored. It is
inefficient.
[0050] The additional measurement frequency is the frequency at
each divided point of (n-1) divided points as a result of dividing
the frequency spacing between the adjacent basic frequency channels
into n divisions. The advantageous effect of the embodiment may be
demonstrated by adopting at least one or more frequencies out of
the frequencies at the (n-1) divided points as the additional
measurement frequencies. When all of the frequencies at the (n-1)
divided points are adopted as the additional measurement
frequencies, the advantageous effect of the embodiment is further
increased. The number of additional measurement frequencies
allocated for each basic frequency channel is preferably
approximately equal to or smaller than the number obtained by
dividing the spacing between the adjacent basic frequency channels
by minimum frequency spacing the user uses. It is inefficient that
the number of additional measurement frequencies is too larger than
the number of frequencies the user uses.
[0051] For example, when approximately 200 basic frequency channels
are set with spacing of 25 GHz in the whole range of C-band,
wavelength calibration is performed on the basic frequency channels
using a wavelength meter and the driving conditions including the
laser driving conditions at the respective basic frequency channels
and the target values of the wavelength detector are recorded in
the memory 370.
[0052] FIG. 4A and FIG. 413 are schematic diagrams each
illustrating a relation between the basic frequency channels and
the additional measurement frequencies. FIG. 4A illustrates four
basic frequency channels. The starting frequency is 191.25 THz and
the spacing between the basic frequency channels is 25 GHz. Basic
frequency channel 1 (.diamond-solid.) is 191.25 THz and additional
measurement frequencies (.diamond.) at five points are allocated
thereto. Basic frequency channel 2 (.box-solid.) is 191.275 THz and
additional measurement frequencies (.quadrature.) at five points
are allocated thereto. Basic frequency channel 3 (.tangle-solidup.)
is 191.3 THz and additional measurement frequencies (.DELTA.) at
five points are allocated thereto. Basic frequency channel 4 ( ) is
191.325 THz and additional measurement frequencies (.largecircle.)
at five points are allocated thereto. The respective additional
measurement frequencies are obtained by wavelength measurement
using the wavelength meter. As illustrated in FIG. 4A, the spacing
between the additional measurement frequencies is not necessarily
set to equal spacing. The additional measurement frequencies
allocated to each basic frequency channel are preferably set
substantially symmetrical with respect to the basic frequency
channel as the additional measurement frequencies exemplarily
illustrated with (.quadrature., .DELTA., .largecircle.). The
additional measurement frequencies may be set only on a side
adjacent to the use frequency as the additional measurement
frequencies exemplarily illustrated with (.diamond.) and allocated
for the end basic frequency channel (191.250 THz). The additional
measurement frequencies allocated to the basic frequency channel
are not limited to be set to the basic frequency channel closest to
the additional measurement frequencies. For example, the additional
measurement frequencies allocated to the basic frequency channel
may be set such that the additional measurement frequencies
allocated for different basic frequency channels are mixed together
in the spacing between the basic frequency channels as the
additional measurement frequencies exemplarily indicated with
(.largecircle.,.DELTA.) Two or more additional measurement
frequencies allocated to the basic frequency channels different
from each other may be the same frequencies. As illustrated in FIG.
4A, the number of divisions for performing the additional
measurement between the adjacent basic frequency channels may
differ from adjacent basic frequency channels to adjacent basic
frequency channels. In addition, as illustrated in FIG. 4B, the
spacing between the basic frequency channels may be different.
[0053] The temperature of the wavelength filter 221 in the
wavelength measurement performed on the additional measurement
frequency is shifted from the temperature of the wavelength filter
221 at the basic frequency channel to which the additional
measurement frequency is allocated in accordance with the
temperature characteristic of the wavelength filter 221. The
wavelength as the result of the measurement and the temperature of
the wavelength filter 221 are stored in the memory 370 as a set of
the measured frequency at the additional measurement frequency and
the temperature of the wavelength filter 221.
[0054] The digital computing unit 360 is a computing unit what is
called a CPU (central processing unit). The digital computing unit
360 calculates appropriate control parameters from data of the
condition of the wavelength tunable light source unit 200 received
from the laser temperature monitoring circuit 321, the PD1 current
monitoring circuit 341, the PD2 current monitoring circuit 342, and
the etalon temperature monitoring circuit 351. The digital
computing unit 360 transmits control signals to the DFB-LD
selection circuit 311, the DFB-LD current control circuit 312, the
laser temperature control circuit 322, the SOA control circuit 330,
and the etalon temperature control circuit 352.
[0055] The digital computing unit 360 is programmed so as to
perform a reference step, a calculation step, and a control step.
In the reference step, the digital computing unit 360 refers to
data of sets of the measured frequencies and the temperatures of
the wavelength filter 221 at two or more points for each basic
frequency channel, the data being stored in the memory 370. In the
calculation step, the digital computing unit 360 selects the basic
frequency channel closest to the frequency of laser light that the
laser light source 210 is instructed to emit, and calculates the
temperature of the wavelength filter 221 for providing the
instructed frequency of laser light from the data of the sets of
the measured frequencies allocated to the basic frequency channel
closest to the instructed frequency and the temperatures of the
wavelength filter 221. In the control step, the digital computing
unit 360 controls the transmission characteristic of the wavelength
filter 221 using the calculated temperature of the wavelength
filter 221.
[0056] The user's instruction of the frequency to the controller
300 via the higher-level controller is not limited to the value of
the frequency. The instruction is given using indirect values in
some cases. Examples of the indirect values include the starting
frequency, the grid spacing, and the number of the channel. The
instructed frequency may be calculated by the following expression
using the indirect values serving as given information.
Instructed frequency-starting frequency+(number of the
channel-1).times.grid spacing
For example, the instructed frequency is calculated to be 191.8168
THz when the starting frequency=191.3 THz, the grid spacing=0.1
GHz, and the number of the channel=5169.
[0057] The following describes a specific example of a method of
calculating the temperature of the wavelength filter 221 for
providing the instructed frequency of laser light using the
temperature of the wavelength filter 221 as a control value of the
wavelength filter 221.
Calculation Method 1
[0058] The digital computing unit 360 selects the basic frequency
channel closest to the instructed frequency of laser light out of
the basic frequency channels stored in the memory 370. The digital
computing unit 360 selects two sets of the measured frequencies
allocated to the basic frequency channel closest to the instructed
frequency and the temperatures of the wavelength filter 221
((Freq1,Tf1) and (Freq2,Tf2)).
[0059] Temperature Tf of the wavelength filter 221 for providing
the instructed frequency of Maser light may be calculated by the
following expression using the two sets of the measured frequencies
and the temperatures of the wavelength filter 221 ((Freq1,Tf1) and
(Freq2,Tf2)).
Tf=Tf1+[(Tf2-Tf1)/(Freq2-Freq1)].times.(Freq_target-Freq1)
where Freq_target is the instructed frequency of laser light.
[0060] As the two sets of the frequencies and the temperatures of
the wavelength filter 221, a set of the frequency of the basic
frequency channel and the temperature of the wavelength filter 221,
and a set of the measured frequency at the additional measurement
frequency and the temperature of the wavelength filter 221 may be
selected. For the calculation, the additional measurement may be
performed on the frequencies at at least one or more divided points
out of the (n-1) divided points between the adjacent basic
frequency channels.
[0061] The calculation method described above employs a simple
linear approximation using the two additional measurement
frequencies. The calculation method may employ a linear
approximation using a least-square method with a number of
additional measurement frequencies. The calculation method is not
limited to employ the linear approximation. The calculation method
may employ a second or higher order polynomial approximation. As
for an interpolation method, interpolation and extrapolation may be
used. The linear approximation or the second or higher order
polynomial approximation may be employed using data of the measured
frequencies interposing the instructed frequency therebetween and
the temperatures of the wavelength filter 221. The linear
approximation or the second or higher order polynomial
approximation may be employed using data of the measured
frequencies that do not interpose the instructed frequency and the
temperatures of the wavelength filter 221.
Calculation Method 2
[0062] In the same manner as calculation method 1, the digital
computing unit 360 selects two sets of the measured frequencies
allocated to the basic frequency channel closest to the instructed
frequency of laser light out of the basic frequency channels stored
in the memory 370 and the temperatures of the wavelength filter 221
((Freq1_a,Tf1_a) and (Freq2_a,Tt2_a)).
[0063] Candidate temperature Tf_a of the wavelength filter 221 for
providing the instructed frequency of laser light may be calculated
by the following expression using the two sets of (Freq1_a,Tf1_a)
and (Freq2a,Tf2a).
Tf_a=Tf1_a+[(Tf2_a-Tf1_a)/(Freq2_a-Freq1_a)].times.(Freq_target-Freq1_a)
[0064] In addition, the digital computing unit 360 selects two sets
of the measured frequencies allocated to the basic frequency
channel secondly closest to the instructed frequency of laser light
out of the basic frequency channels stored in the memory 370 and
the temperatures of the wavelength filter 221 ((Freq1_b,Tf1_b) and
(Freq2_b,Tf2_b)). In the same manner as the calculation of Tf_a,
candidate temperature Tf_b of the wavelength filter 221 for
providing the instructed frequency of laser light may be calculated
by the following expression.
Tf_b=Tf1_b+[(Tf2_b-Tf1_b)/(Freq2_b-Freq1_b)].times.(Freq_target-Freq1_b)
[0065] From two candidate temperatures Tf_a and Tf_b, the one
closer to the center value of the temperature of the wavelength
filter 221 stored in the memory 370 is adopted as the temperature
of the wavelength filter 221 for providing the instructed frequency
of laser light.
[0066] In the same manner as calculation method 1, the calculation
method described above may employ a linear approximation using a
least-square method with a number of additional measurement
frequencies and a second or higher order polynomial approximation.
As for the interpolation method, interpolation and extrapolation
may be used.
[0067] FIG. 5 is a schematic diagram explaining an effect of
calculation method 2. In the graph illustrated in FIG. 5, the
abscissa axis represents the frequency, the ordinate axis
represents the temperature of the wavelength filter 221, and the
instructed frequency of laser light is 191.2865 THz. In the graph,
plotted are the sets of the measured frequencies allocated to basic
frequency channel 1 (191.275 THz) closest to the instructed
frequency of laser light and the temperatures of the wavelength
filter 221, and the sets of the measured frequencies allocated to
basic frequency channel 2 (191.3 THz) secondly closest to the
instructed frequency of laser light and the temperatures of the
wavelength filter 221. The spacing between the basic frequency
channels is 25 GHz. The center value of the temperature of the
wavelength filter 221 stored in the memory 370 is 54.degree. C.
[0068] Specifically, additional measurement data 1 (+) at five
points are the sets of the measured frequencies allocated to basic
frequency channel 1 closest to the instructed frequency and the
temperatures of the wavelength filter 221, and additional
measurement data 2 (.DELTA.) at five points are the sets of the
measured frequencies allocated to basic frequency channel 2
secondly closest to the instructed frequency and the temperatures
of the wavelength filter 221. The wavelength filter temperature ( )
is the wavelength filter temperature at the instructed frequency of
laser light calculated by interpolating the additional measurement
data 1 (+) while the wavelength filter temperature (.largecircle.)
is the wavelength filter temperature at the instructed frequency of
laser light calculated by extrapolating the additional measurement
data 2 (.DELTA.).
[0069] As may be seen from FIG. 5, the wavelength filter
temperature (.largecircle.) calculated from the additional
measurement data (.DELTA.) of basic frequency channel 2 secondly
closest to the instructed frequency is closer to the center value
(=54.degree. C.) of the temperature of the wavelength filter 221
than the wavelength filter temperature ( ) calculated from the
additional measurement data (+) of basic frequency channel closest
to the instructed frequency. In this case, calculation method 2
adopts the wavelength filter temperature (.largecircle.) calculated
from the additional measurement data (.DELTA.) of basic frequency
channel 2 secondly closest to the instructed frequency as the
temperature of the wavelength filter 221 for providing the
instructed frequency of laser light. As described above,
calculation method 2 adopts the wavelength filter temperature
closer to the center value of the temperature of the wavelength
filter 221 as the temperature of the wavelength filter 221 for
providing the instructed frequency of laser light. When the center
value of the temperature of the wavelength filter 221 stored in the
memory 370 is preliminarily set to an appropriate value, a low
temperature that causes power consumed by the Peltier element 222
to be increased is not adopted as the temperature of the wavelength
filter 221. As a result, power consumed by the Peltier element 222
may be kept low. The example of the calculation method of the
filter temperature illustrated in FIG. 5 uses only the additional
measurement data of the additional measurement frequencies
allocated to the basic frequency channels and the temperatures of
the wavelength filter 221. The data of the frequencies of the basic
frequency channels and the temperatures of the wavelength filter
221 may be used besides the additional measurement data.
Control Method
[0070] The following describes the whole flow of a control method
of the wavelength tunable laser module 100 with reference to FIG.
6. FIG. 6 is a schematic diagram illustrating an example of the
whole flow of the control method of the wavelength tunable laser
module.
[0071] As described above, the data including the number and the
temperature of the DFG-LD 211, the temperature of the wavelength
filter 221, the current supplied to the DFB-LD 211, and the PD
current ratio (=PD1/PD2) serving as a feedback control target value
is stored as the initial value for each basic frequency channel in
the memory 370 of the controller 300. The data is acquired by the
wavelength calibration using the wavelength meter and recorded in
the memory 370 prior to the shipment of the wavelength tunable
laser module.
[0072] In addition, in the memory 370 of the controller 300, the
data is stored that includes frequencies at five points and the
respective temperatures of the wavelength filter 221 (the data of
the frequency of the basic frequency channel and the temperature of
the wavelength filter 221, and the data of the measured frequencies
at the four additional measurement frequencies and the respective
temperatures of the wavelength filter 221) for each basic frequency
channel, for example. The data is acquired by the wavelength
measurement using the wavelength meter and recorded in the memory
370 prior to the shipment of the wavelength tunable laser
module.
[0073] At step S1, the controller 300 calculates the target value
of the temperature of the wavelength filter 221 in accordance with
the frequency instructed by the user via the higher-level
controller. Specifically, the digital computing unit 360 of the
controller 300 refers to the data of the set of the measured
frequencies and the temperatures of the wavelength filter 221 at
two or more points for each basic frequency channel, the data being
stored in the memory 370, selects the basic frequency channel
closest to the frequency of laser light instructed by the user to
cause the laser light source 210 to emit, and calculates the
temperature of the wavelength filter 221 for-providing the
instructed frequency of laser light from the data of the sets of
the measured frequencies allocated to the basic frequency channel
closest to the instructed frequency and the temperatures of the
wavelength filter 221.
[0074] At step S2, controlling the temperatures of the DFB-LD 211
and the wavelength filter 221 starts.
[0075] The digital computing unit 360 of the controller 300
transmits the control signal to the etalon temperature control
circuit 352 so as to cause the temperature of the wavelength filter
221 to reach the calculated target value, and starts monitoring the
temperature of the wavelength filter 221 via the filter temperature
monitoring element 223 and the etalon temperature monitoring
circuit 351.
[0076] As a result, as illustrated in FIG. 7, the discrimination
curve corresponding to the frequency instructed by the user is
obtained that is shifted from the discrimination curve of the
wavelength filter 221 at the basic frequency channel in the
wavelength direction. FIG. 7 is a schematic diagram illustrating an
example of the shifting of the discrimination curve in the
wavelength direction. In FIG. 7, the discrimination curve at the
basic frequency channel is indicated with the broken line while the
discrimination curve corresponding to the frequency instructed by
the user is indicated with the solid line.
[0077] At step S3, the DFB-LD 211 corresponding to the basic
frequency channel is selected out of the multiple DFB-LDs 211 and
supplying a constant current to the selected DFB-LD 211 starts.
Specifically, the digital computing unit 360 of the controller 300
transmits the respective control signals to the DFB-LD selection
circuit 311 and the DFB-LD current control circuit 312.
[0078] At step S4, the digital computing unit 360 of the controller
300 stands by until both of the temperatures of the DFB-LD 211 and
the wavelength filter 221 are in respective set ranges.
Specifically, the digital computing unit 360 of the controller 300
monitors temperature information from the laser temperature
monitoring circuit 321 and the etalon temperature monitoring
circuit 351 and determines whether the temperatures of the DFB-LD
211 and the wavelength filter 221 are in the respective set
ranges.
[0079] If both of the temperatures of the DFB-LD 211 and the
wavelength filter 221 are in the respective set ranges, supplying a
current to the SOA 214 starts and auto power control (AFC) starts
at step S5. Specifically, the digital computing unit 360 of the
controller 300 transmits the control signal to the SOA control
circuit, and performs feedback control such that the current from
the power monitor 240 reaches the target value stored in the memory
370 while monitoring the power of the laser light source 210 via
the power monitor 240 and the PD1 current monitoring circuit
341.
[0080] As illustrated in FIG. 8, with an increase in current
supplied to the SOA 214, the current output by the power monitor
240 is increased. FIG. 8 is a graph illustrating a relation between
the current supplied to the SOA and the current output by the power
monitor. At step S6 and step S7, the power of the laser light
source 210 may be adjusted to the target value while the power of
the laser light source 210 is increased and whether the power of
the laser light source 210 reaches the target value is monitored
using the relation illustrated in FIG. 8.
[0081] If the power of the laser light source 210 reaches the
target value, auto frequency control (AFC) is performed at step S8.
Specifically, the digital computing unit 360 of the controller 300
monitors the current (PD1) output from the power monitor 240 via
the PD1 current monitoring circuit 341 and the current (PD2) output
from the wavelength monitor 250 via the PD2 current monitoring
circuit 342, and calculates the PD current ratio (PD2/PD1).
[0082] The digital computing unit 360 of the controller 300
performs feedback control such that the calculated PD current ratio
becomes the PD current ratio stored in the memory 370 as the
feedback control target value of the basic frequency channel on the
basis of the discrimination curve illustrated in FIG. 9. FIG. 9 is
a graph illustrating an exemplary discrimination curve that
corresponds to the instructed frequency and is obtained by
controlling the temperature of the wavelength filter 221 so as to
become the etalon temperature target value obtained at step S1.
[0083] The discrimination curve used for the AFC at step S8 is
obtained by shifting the discrimination curve at the basic
frequency channel in the wavelength direction. The feedback control
target value used for the AFC at step S8 may use the PD current
ratio stored in the memory 370 as the feedback control target value
of the basic frequency channel.
Accuracy Verification
[0084] FIG. 10 is a graph illustrating an exemplary measurement
result of wavelength accuracies in the wavelength control method of
the wavelength tunable laser module according to the embodiment.
FIG. 11 is a graph illustrating an exemplary measurement result of
wavelength accuracies in the wavelength control method of the
wavelength tunable laser module as a comparative example. In the
measurement result of the wavelength accuracies illustrated in FIG.
10, the wavelength accuracy is measured with a spacing of 0.1 GHz
when C-band is divided by a spacing of 25 GHz to set basic
frequency channels and the additional measurement frequencies are
set by dividing the spacing between the adjacent basic frequency
channels into six divisions in the embodiment. In the measurement
result of the wavelength accuracies illustrated in FIG. 11, the
wavelength accuracy is measured with a spacing of 0.1 GHz in a
method that determines the temperature of the wavelength filter (a
shift amount of the discrimination curve) from the temperature
characteristic of the wavelength filter. The wavelength accuracies
are measured using the same wavelength meter for both cases
illustrated in FIGS. 10 and 11.
[0085] As may be seen from the comparison between FIGS. 10 and 11,
the wavelength accuracy is approximately .+-.0.6 GHz in the
comparative example. In the embodiment, the wavelength accuracy is
approximately .+-.0.2 GHz, which is more accurate than that in the
comparative example. The method of controlling the wavelength of
the wavelength tunable laser module according to the embodiment may
perform control on all of the use frequencies without preliminarily
storing the control parameters for all of the use frequencies. In
addition, the method of controlling the wavelength of the
wavelength tunable laser module according to the embodiment may
increase the wavelength accuracy. With the present disclosure, a
method of controlling the wavelength of the wavelength tunable
laser module that may perform control on all of the use frequencies
without preliminarily storing control parameters for all of the use
frequencies may be provided.
[0086] The embodiment to which the disclosure made by the inventor
is applied has been described above. The description and the
drawings, which partially constitute of the disclosure of the
embodiment, however, does not limit the disclosure. In the
embodiment, the semiconductor laser elements formed in an array are
used for the laser light source. A structure that includes no
optical multiplexer and no semiconductor optical amplifier may be
used, for example. A single longitudinal mode semiconductor laser
element such as a distributed Bragg reflector (DBR) laser element
may be used, for example. In this way, other embodiments, examples,
and operation techniques carried out by the skilled person in the
art on the basis of the embodiment are all included in the scope of
the disclosure.
[0087] Although the disclosure has been described with respect to
specific embodiments for a complete and clear disclosure, the
appended claims are not to be thus limited but are to be construed
as embodying all modifications and alternative constructions that
may occur to one skilled in the art that fairly fall within the
basic teaching herein set forth.
* * * * *